Project summary Life requires maintenance of the blood and interstitial fluids within a narrow range of physical parameters, and an increase in plasma osmolarity of only 1-2% stimulates intense thirst as well as hormonal mechanisms that promote sodium excretion and water retention by the kidney. This coordinated response is thought to originate from specialized osmosensitive neurons in the brain. However, the molecular and cellular mechanisms responsible for central osmosensation remain poorly defined. Recently, a specific population of excitatory neurons within the subfornical organ (SFO) of the brain defined by expression of the gene Nos1 was shown to be necessary and sufficient for the control of drinking behavior in mice, and recordings of the activity of these neurons in awake, behaving mice revealed that they are rapidly and dose-dependently activated by increases in blood osmolarity. I propose here to systematically investigate the mechanism by which SFO neurons detect changes in the blood osmolarity. I will examine whether these neurons sense blood osmolarity directly or through an intermediary circuit mechanism and will seek to identify the specific neural or molecular components that are necessary for this process. Central osmosensation plays an important role in cardiovascular fitness, and goes awry in cases of hypertension, stroke, and cardiovascular disease. The proposed experiments may reveal therapeutic targets for such conditions. Additionally, this work will deepen our understanding of the neural circuit that regulates thirst and may also illuminate more general molecular- and circuit-mechanisms by which the brain monitors the state of the body.